Preparation Of Regenerated Cellulose Nanofiltration Membranes And Their Application In The Depth Treatment Of Drinking Water

Presently,commercial nanofiltration membrane is basically not biodegradable petrochemical products,and it brings a huge burden to the environment.There is an urgent need to develop environmentally friendly nanofiltration membranes to replace it.Cellulose is the most abundant natural polymer.It is a renewable resource consisting with sustainable development.It has the advantages of wide applications excellent physical and chemical properties,good hydrophilicity,good biocompatibility,wide source,renewable and low cost.It is an ideal material for replacing petroleum polymer membrane.In this study,cellulose membrane and cellulose/chitosan membrane were prepared using N-methylmorpholine-N-oxide(NMMO)as solvent,bamboo dissolving pulp and chitosan as raw materials.The cellulose nanofiltration membrane,cellulose/chitosan antibacterial nanofiltration membrane and interfacial polymeric cellulose/chitosan composite nanofiltration membrane were studied by alkali hydrolysis,carboxymethylation modification and interfacial polymerization,respectively.The mechanism of membrane formation through casting solution,modification and interfacial polymerization were studied.In this paper,the influence factors,such as mechanical properties,microstructure,chemical composition,crystal structure,thermal stability,antibacterial properties and separation properties were studied.The interfacial polymeric cellulose/chitosan composite nanofiltration membrane was selected for the deep treatment of drinking water,and the pollution mechanism of the nanofiltration membrane during the deep treatment were explored.This study provides the theoretical basis and scientific basis for the industrial application of regenerated cellulose nanofiltration membrane for deep processing of drinking water.In this paper,bamboo dissolving pulp was firstly dissolved(6 wt%)in NMMO.The cellulose membrance was prepared by phase inversion method.Cellulose nanofiltration membrane was prepared by sodium hydroxide hydrolysis and carboxymethylation of chloroacetic acid with dense cellulose membrane as base membrane.The mechanism of this reaction was SN2 nucleophilic substitution.At the operating pressure of 0.5MPa,the retention rates of the cellulose nanofiltration membrane on the concentration of 500 mg/L NaCl and Na2SO4 aqueous solution were 34.9%and 68.4%,the water flux were 13.12 L/m2·h and 10.32 L/m2·h,respectively.The retention rates of the 10 mg/L methyl orange and methyl blue aqueous solutions were 93.0%and 98.9%,and the water flux were 12.31 L/m2·h and 10.12 L/m2·h,respectively.The retention molecular weight of the cellulose nanofiltration membrane is 678 Da and the Stokes radius is about 0.63 nm.The hydrogen bond in the cellulose membrane is destroyed.The lattice was swollen and scattered in some degress.At the same time,a large number of aqueous solutions passed into the cellulose membrane,causing intense swelling,and cross-sectional and longitudinal shrinkage.As a result some cracks and holes appeared in the cellulose membrane and formed effective channel.In the FT-IR spectrum of cellulose nanofiltration membrane,the 1752.6 cm-1 was attributed to the C=O absorption peak.It indicated that the cellulose membrane was modified by alkaline hydrolysis and chloroacetic acid in the carboxymethylation reaction then part of hydroxyl group was replaced by the carboxymethyl groups.In the process of dissolution and regeneration of cellulose,intermolecular hydrogen bond was opened,and made the conversion of cellulose I to cellulose ?,the cellulose membrane and cellulose nanofiltration membrane appears only a diffraction peak,showed that cellulose membrane modification occurs mainly in the amorphous region,cellulose membrane and cellulose nanofiltration membrane diffraction peak intensity is significantly weaker than cellulose diffraction peak intensity.The calculation of the crystallinity of cellulose was 71.2%,the crystallinity of cellulose membrane is 62.3%,the crystallinity of cellulose nanofiltration membrane is 60.1%.The order of thermal stability was cellulose>cellulose membrane>cellulose nanofiltration membrane,mainly due to intermolecular hydrogen bond in the cellulose dissolving process was opened.After solidified membrane forming hydrogen bond was not fully connected and the regenerated cellulose polymerization degree declined.The adhesion strength of the modified carboxymethyl was weaker than the adhesion strength of the hydrogen bonds.In this paper,the 6 wt%of bamboo dissolving pulp and chitosan were firstly dissolved in NMMO and then formed the cellulose/chitosan membrane by phase inversion method.The cellulose/chitosan antibacterial nanofiltration membrane was prepared by hydrolysis and carboxymethylation modification.The mechanical properties of chitosan macromolecular is week,and macromolecular main chain on the six-member ring structure is not likely to rotate.In addition,when the chitosan dissolves,the intermolecular bond binding force is abate,causing decrease of cellulose/chitosan nanofiltration membrane tensile strength.In cellulose/chitosan blend,cellulose is the main force bearer,rending the blend membrane tough and less brittle.The mechanical properties of cellulose/chitosan nanofiltration membrane was mainly decided by the cellulose.In the FT-IR spectrum,the bending vibration peak of antibacterial groups(-NH)belongs to the structure of chitosan in the cellulose/chitosan nanofiltration membrane.The carboxymethyl C=O peak indicated that the hydroxyl was replaced by carboxymethyl in the modification process part.The crystallinity of cellulose/chitosan nanofiltration membrane was between cellulose and chitosan.In the process of dissolution and regeneration of cellulose and chitosan,the hydrogen bonds were reconstructed.The crystallization of blend membrane structure was formed by interaction of chitosan and cellulose.The order of thermal stability was as following:cellulose>cellulose/chitosan nanofiltration membrane>chitosan.The intermolecular hydrogen bonds between the cellulose and chitosan were destroyed by NMMO hydrate when dissolved.The intramolecular hydrogen bonding was formed after solidification regeneration process.As a result,the cellulose and chitosan binding force was abate,which affecting their thermal stability.With the increase of chitosan content,the hydrophilic property of nanofiltration membrane enhanced.The intermolecular hydrogen bond was destroyed when cellulose and chitosan dissolved in NMMO.The cellulose-chitosan hydrogen bond was formed in the solid regeneration process.The internal structure became loose,rending the nanofiltration membrane hydrophilic.With the increase of chitosan content,the antimicrobial properties of cellulose/chitosan nanofiltration membrane was enhanced.The ratio of cellulose to chitosan was 6:1,at the operating pressure of 0.5 MPa,the retention rates of cellulose/chitosan antibacterial nanofiltration membrane at the concentration of 500 mg/L NaCl,Na2SO4 and MgSO4 were 34.42%,67.71%and 66.83%,respectively.The water flux were 13.51 L/m2·h,12.12 L/m2·h and 12.43 L/m2·h,respectively.The rejection rates of 100 mg/L methyl orange and methyl blue aqueous solution were 92.37%and 98.79%,respectively.The water flux were 13.64 L/m2·h and 12.37 L/m2·h,respectively.The trapped molecular weight of cellulose/chitosan antibacterial nanofiltration membrane was 716 Da and Stokes radius was about 0.65 nm.In this paper,cellulose/chitosan composite nanofiltration membrane was prepared by interfacial polymerization using piperazine as the aqueous monomer and trimesic acid chloride as the organic monomer.The optimum conditions were as following:the concentration of PIP in the aqueous phase was 2 wt%,the treatment time was 30 min,the TMC concentration was 0.15 wt%,and the reaction time was 3 min.At a concentration of 500 mg/L NaCl and Na2SO4 aqueous solution at an operating pressure of 0.5 MPa,the intertwined cellulose/chitosan composite nanofiltration membrane had a rejection rate of 40.26%and 71.34%,respectively,and a water flux were 15.64 L/m2·h and 13.56 L/m2·h,respectively.The rejection rates of 100 mg/L methyl orange and methyl blue aqueous solution were 93.65%and 98.86%,respectively.The water flux were 13.28 L/m2·h and 12.35 L/m2·h,respectively.The interfacial polymeric cellulose/chitosan composite nanofiltration membrane has a molecular weight of 823 Da and a Stokes radius of about 0.7 nm.The order of the intervening polymeric cellulose/chitosan composite nanofiltration membrane was:R(Na2SO4)>R(MgSO4)>R(MgCl2)>R(NaCl).The composite nanofiltration membrane surface is negatively charged,and the retention characteristics of inorganic salts and dyes are the result of the combination of electrostatic effect and sieving effect.In the FT-IR spectrum of IP-BC/CS-NFM,1635.1 cm-1 was the characteristic peak of amide(-CONH-),and 3444.4 cm-1 was the absorption peak of piperazine ring-NH,and 1432.3 cm-1 was the vibration characteristic peak of the benzene ring,which proves that polypiperazine amide had been generated in the nanofiltration membrane.The FT-IR spectrum of IP-BC/CS-NFM characteristic peak in 1160.2 cm-1,showed that a parts of cellulose hydroxy acylation reaction with TMC.Through the physical and chemical interaction,a uniform and dense layer was formed on the cellulose/chitosan matrix membrane.The polyamide functional layer rend the combination of the copolymerization layer and basal membrane more solid.The surface microstructure of the base membrane(cellulose/chitosan membrane)was uniform and the spongy porous network structure was presented.After interfacial polymerization PIP with TMC,basal membrane on the surface of the hole was filled with aggregates of polyamide,and generated on the surface of a layer of basement membrane,which made the IP-BC/CS-NFM interception of nanofiltration characteristics.The polyamide layer formed by interface polymerization was inlaid into the surface of the basement membrane through the combination of physics and chemistry,so that the copolymers were not easy to fall off.In the heating process of cellulose/chitosan membrane from 263.1? and 355.3?,there was a big weight loss of about 50.18%.From from 281.4? to 355.3? there was a big weight loss of about 48.95%in the IP-BC/CS-NFM.The thermal stability of IP-BC/CS-NFM was better than cellulose/chitosan membrane.The effects of three kinds of nanofiltration membranes on the removal of chloroform,nitrate nitrogen and hardness were investigated.The effects on the removal of trichloromethane,nitrite nitrogen and hardness were investigated for the three kinds of nanofiltration membrane.The removal order were as following,interfacial polymerization of cellulose/chitosan composite nanofiltration membrane>cellulose nanofiltration membrane>cellulose/chitosan antimicrobial nanofiltration membrane.All the three kinds of nanofiltration membrane can remove the pollutants and meet the state drinking water safety standards.At the same time,it was found that the removal of chloroform of the cellulose nanofiltration membrane and cellulose/chitosan antibacterial nanofiltration membrane was higher in the neutral medium than the acidic or alkaline conditions.For the interfacial polymerization of cellulose/chitosan composite nanofiltration,the removal rate of chloroform in acidic conditions was lower than that in neutral,but the removal rate was similar to that of neutral under alkaline condition.With the increase of the influent temperature,the removal rate of trichloromethane was gradually reduced.The interception rate of the three nanofiltration membranes to the chloroform solution was basically stable by continuous operation and intermittent operation.In this paper,the water treated with polyvinylidene fluoride(PVDF)ultrafiltration membrane and the interface polymerized cellulose/chitosan composite nanofiltration membrane was used to treat the precipitated water from the water plant.In this study,the CODMn index fluctuates with time,the average removal rate of CODMn is 34.7%.The basic control of CODMn in nanofiltration is below 1.0mg/L.The turbidity of nanofiltration is 0.01NTU-0.04NTU.Ultrafiltration membrane of raw water in the iron ions basically showed no retention capacity.The removal rate of iron ions from the nanofiltration membrane was between 67.3%and 76.4%.The concentration of iron ions in nanofiltration was less than 0.21 mg/L.The ultrafiltration on the chloride and sulfate basically showed no ability to retain.The average concentration of chloride and sulfate in water from nanofiltration were 179.1 mg/L and 88.5 mg/L,respectively.The water quality was stable.The total number of colonies in the effluent after the ultrafiltration treatment and the total coliform index were not detected.The indicators were lower than the "drinking water health standards" GB5749-2006.In the ultrafiltration-nanofiltration advanced treatment of drinking water,the effect of different fouling on the flux decay in the nanofiltration membrane was quantified by the series resistance model and the flux decay rate.Theoretical analysis and experimental results show that,the flux attenuation of the nanofiltration membrane was mainly caused by the concentration polarization and the adsorption fouling.